Chart puzzle 1...where in Maine?

[Dan Foster]

Where's this? For extra credit, what's the current magnetic declination?

[Nancy Hill]

Joe's Middle Ground: It's the channel North of Machias Bay heading into the Machias River. Near that area (around Holmes Bay and Birch point) are ancient petroglyphs, but I have not been able to find them.

[gyork]

3 hours ago, Dan Foster said:

Where's this? For extra credit, what's the current magnetic declination?

My thoughts:

Based on depth, far, far away from Frenchman Bay, due S Mag. Can't seem to match up depth and buoy.

Based on today's date, and depending on when in 2018 the reference date, at a minimum decrease (12.31.18) of 6.35' and a maximum decrease (1.1.18) of 11.35, I've come up with a range between 15°53.65' and 15°49.65'. I need some education here.

[Nancy Hill]

Solution to the "hard" one from Dan: West side of Penobscot Bay, between Rockland and Camden Harbors. Dan, do you mean variation? If so, the variation on your chart is from 2018 (fairly recent), so subtracting 5' per year would be a variation of 15 degrees 50' W for 2020.

[Pintail]

He <probably> meant variation; but he <might> have wanted to know the declination...you never know...

By the way, could this not simply be an anomaly?  Or a misprint?

<I still can't wrap my head around this peculiar feature, but I'll still work on it>

Edited April 8, 2020 by Pintail

[Nancy Hill]

21 minutes ago, Pintail said:

He <probably> meant variation; but he <might> have wanted to know the declination...you never know...

Ok, now I remember the terms variation and declination are used interchangeably when referring to true versus magnetic north.  When I see the word declination, I think of measuring the angle (with a sextant) of an object above the horizon. I have never used those words interchangeably, but see how they could be since they both measure angles. Just don't anyone call it deviation! I will really roll my eyes. ?

[gyork]

4 hours ago, gyork said:

My thoughts:

Based on depth, far, far away from Frenchman Bay, due S Mag. Can't seem to match up depth and buoy.

Based on today's date, and depending on when in 2018 the reference date, at a minimum decrease (12.31.18) of 6.35' and a maximum decrease (1.1.18) of 11.35, I've come up with a range between 15°53.65' and 15°49.65'. I need some education here.

Playing around a bit with my online nautical charts, I've come to realize that the compass rose, as displayed on Caltopo, only shows differences of variation of 0.25', as I cursor-creep down the coastline. This would account for my answer above being not a little off! Still a bit shocked to see a depth of 488 in PenBay!

[gyork]

1 hour ago, Nancy Hill said:

........ the terms variation and declination are used interchangeably when referring to true versus magnetic north.  

topographic MAPS-declination; nautical CHARTS-variation (preferred)

[Dan Foster]

Well done, Nancy, with a correct solution to the compass rose teaser above. My apologies for using the land-lubber's magnetic declination when you all expected variation. In my defense, I was on land when I wrote it.

Mini-game within this thread: fix the land-lubber's phrases below with the more correct nautical terminology:

1. High-speed ferry approaching from the left!

2. The white thing is flapping again, tighten up the rope!

3. Did you really just steal toilet paper from the bathroom? Let's work this out in a civilized fashion.

 

My answers (having never owned a sailboat or lost my leg climbing up the top sails):

1. High-speed ferry approaching from starboard!

2. The mizzen yard's a luffin', haul on that line, matey!

3. Arrgh!, Ye be thieving clam shells from the head, ya scurvy bilge rat? Keelhaul him, boys, and lash him to the poop deck 'til we round the Isles of Costco.

 

Looking forward to reading your declinations, I mean, variations.

[gyork]

On 4/6/2020 at 8:17 AM, gyork said:

 I still can't wrap my head around this peculiar feature, but I'll still work on it.  

 

From the local Harbormaster:

I have always wondered myself why that deep spot exists. My only conclusion was that there is a large amount of water flowing down the New Meadows River on the East side of the bridge, and the Long Reach is attached to Harpswell Sound/Ewing Narrows on the west side of the bridge. When the tides change, the current is VERY strong. I have wondered if the hydraulics formed there, at an apex, and that’s what caused the hole after many years. We often worry about shellfish harvesters “freighting” their skiffs with clams because if they get sideways on the incoming tide under the bridge, its likely to sink the skiff.

 I’ve been actively boating/driving this area regularly for 10 years and have not seen a typical “whirlpool”. Although it does tend to have whirlpool characteristics at the change of every tide, just on a smaller scale.

 Other than my own speculation, I don’t have any scientific knowledge to explain that spot. If you find an answer, feel free to keep us posted as I’m sure it’s interesting.

 

Edited April 13, 2020 by gyork

[Joseph Berkovitz]

I think this is likely an example of hydrodynamic scour.

This is a bottom phenomenon not a surface phenomenon so one wouldn’t necessarily see a whirlpool up top. It occurs because of vortices of rotating water below the surface that occur at a point of obstructions. Often this can be bridge pilings (see bridge scour) but it can occur at any point where there’s an obstruction that spins off rotating currents. In this case, that would be the point just below the bridge. All you need is a mechanism that takes away bottom sediment bit by bit and drops it somewhere else. These holes can become very deep According to the article. 

[Nancy Hill]

1 hour ago, Joseph Berkovitz said:

I think this is likely an example of hydrodynamic scour.

Thanks, Joe. I'm glad to know the term. This must be what is happening when I stand in shallow water and the waves wash over my feet and I sink deeper into the sand as the motion of the water removes sand from the edges of my feet.

[mhabich]

Joe, really cool.  Looking at the bridge scour article, it seems that contraction scour would occur at the contraction (under the bridge), not after.  I wonder if the bottom at the natural contraction where the bridge was built is rock, with the area around the hole might be sediments that filled in after the glacier scraped out the north-south basin.  So, a combination of the scour mechanism acting on peculiar pre-existing geology.

[Joseph Berkovitz]

i bet you are right about there being a deep bed of post glacial sediment there. Specifically, at least 90 feet worth!

Just To be clear, I didn’t mean that the bridge itself was causing this, just that the phenomenon is similar to bridge scour. But in this case I suspect the hole is caused by an eddy directly off the point where long reach meets doughty cove. The bridge seems too far away to be the cause. From the configuration of the hole just upstream of the point (including its odd little tail), I’m guessing the scouring takes place on the flood not the ebb.

[JohnHuth]

Gary asked me about this via e-mail, and I'm just checking in.  One thought is that it's a submerged kettle that was once exposed when sea levels were lower.   What is the bottom like there?  Sediment or rocky?  If rocky, that might eliminate a scour, and if it's mucky sediment, that might eliminate a kettle hole. 

 

 

[gyork]

Here's snippet chart #7. 1. Where is it?  2. When did you paddle here?